In recent years, gas tanks (gas cylinders) that store hydrogen or natural gas serving as fuel for electric power generation have been used in automobiles, houses, transport machinery, and the like.
For instance, polymer electrolyte fuel cells have been gaining attention as a power source for automobiles. When such fuel cells are used for electric power generation, an electrochemical reaction is induced by supplying a gas fuel (e.g., hydrogen gas) to a gas diffusion electrode layer provided on one side of each fuel cell and supplying an oxidant gas (e.g., air containing oxygen) to a gas diffusion electrode layer provided on the other side. Upon such electric power generation, nontoxic water is exclusively produced. Thus, the above fuel cells have been gaining attention from viewpoints of environmental influences and use efficiency.
In order to continuously supply a gas fuel such as hydrogen gas to an automobile equipped with the above fuel cells, a gas fuel is stored in an in-vehicle gas tank. Examples of in-vehicle hydrogen gas tanks that have been examined include a gas tank that stores compressed hydrogen and a hydrogen-storing gas tank that stores hydrogen in a state of absorption in metal hydride (MH).
Among them, a CFRP (carbon fiber-reinforced plastic) tank has been examined to be used as an in-vehicle gas tank that stores compressed hydrogen. A CFRP tank is structured such that a liner layer (inner shell) that maintains airtight properties of the tank is formed inside a layer (outer shell: fiber-reinforced layer) comprising a carbon fiber-reinforced plastic (CFRP material). Such CFRP tank has strength greater than that of a tank made of a usual type of plastic and is excellent in pressure resistance, so that it is preferably used as a gas fuel tank.
In addition, a known method for producing a CFRP tank involves a technique for producing a CFRP tank by winding a CFRP material in a carbon fiber filament form around a liner layer that is formed in a container form (filament winding method). Since carbon fibers have strength and rigidity in the fiber direction, the strength of a tank can be improved by winding a CFRP material in the manner described above.
Meanwhile, for fuel storage purposes, a pressure-resistant tank is mounted in an automobile for which a liquefied gas such as high-pressure gas (natural gas) or propane is used as fuel. In general, commercially available and widely used pressure-resistant tanks are made of metals such as steel and aluminium. However, metal-made pressure-resistant tanks are thick and heavy. Thus, in addition to poor workability and characteristics that cause danger, great energy is required for transport of such tanks, resulting in an automobile mileage decrease. This is disadvantageous. Further, the calorific value per unit weight of gas fuel is almost half that of gasoline. Thus, in order to extend the distance that a gas automobile can run without fuel supply to an extent comparable to the case of a commercially available gasoline-fueled automobile, it is necessary to supply a gas fuel with a weight greater than that of gasoline, which is problematic.
Thus, in order to achieve weight reduction, a gas tank having an aluminium- or plastic-made inner shell and an outer shell that covers the inner shell and is made of pressure-resistant FRP (fiber-reinforced plastic) has been developed. Such gas tank is essentially made of plastic and thus is much lighter in weight than a metal-made gas tank. Therefore, it is expected that mileage can be improved with the use of the above gas tank as a natural gas tank for automobiles. However, the weight of the outer shell accounts for the most of the weight of a tank. Accordingly, a tank comprising an outer shell having a minimized weight is preferable because such tank is lighter in weight than other tanks. Also, with the use of such tank, in addition to the improvement in mileage, reduction in wear-and-tear expenses for abrasion of tires and brake shoes, laborsaving handling of cylinders, and reduction in accidents can be expected.
However, FRP is more fragile than metals, and thus it might experience generation of cracks and the like when receiving great impact force from the outside. Crack propagation might result in sharp reduction in the pressure resistance and the strength of an FRP-made outer shell. In addition, even when there is slight damage to appearance, cracks and damages in reinforcing fibers are extended due to repetitive application of impact force, which might result in reduction in pressure resistance and strength.
Thus, JP Patent Publication (Kokai) No. 8-219386 A (1996) discloses a gas tank having a gas-barrier inner shell and an outer shell that is formed so as to cover the inner shell and is made of a pressure-resistant FRP, such outer shell comprising reinforcing fiber bundles [A], a hardened material made of a thermosetting resin [B], and an elastomer and/or thermoplastic resin [C], provided that the elastomer and/or thermoplastic resin [C] is localized on the outer circumference of the reinforcing fiber bundle [A] in a cutting section of the outer shell. Such gas tank has been realized in order to impart toughness to the FRP-made outer shell, to maintain the high-pressure resistance, and to suppress propagation of cracks and damages in reinforcing fibers so as to improve impact resistance and fatigue resistance. The gas tank disclosed in JP Patent Publication (Kokai) No. 8-219386 A (1996) is based on technology for realization of a CNG pressure container. According to such technology, the toughness of a matrix resin is improved in a manner such that crack extension in FRP induced by an impact is prevented on the assumption that an impact is applied to FRP and that an impact is repeatedly applied to FRP. As a means of improving toughness, a thermoplastic elastomer is used. The desired strength can be obtained particularly with the use of a polyester or polyamide elastomer among thermoplastic elastomers.
However, in the case of a gas tank disclosed in JP Patent Publication (Kokai) No. 8-219386 A (1996), an elastomer and/or thermoplastic resin [C] is localized on the outer circumferences of reinforcing fiber bundles [A], resulting in the following problems.
(1) Delamination occurs due to poor compatibility between a thermosetting resin serving as a base resin containing a thermoplastic elastomer and fibers. This is because a thermoplastic elastomer is not sufficiently dispersed but is localized in a base resin due to a difference between a thermoplastic resin used as an elastomer and a thermosetting resin used as a base resin, so that stable properties cannot be obtained.(2) The use of a thermoplastic elastomer results in poor impregnation of fibers with such elastomer upon filament winding (FW) molding of a high-pressure tank. This is because introduction of a thermoplastic elastomer results in increased viscosity of a resin used for FW, leading to poor impregnation of fibers with such elastomer.(3) Addition of a thermoplastic elastomer results in a decreased glass transition point (Tg) of a CFRP matrix itself. Accordingly, the heat resistance decreases. In addition, the acceptable heat resistance of a tank used as a container is not specifically described. The glass transition temperature of an elastomer component is generally low and thus the heat resistance originally imparted to a base resin cannot be maintained. Further, high-temperature cycle tests are carried out as environmental tests for a container. Thus, it is necessary to describe the acceptable heat resistance of a matrix resin.(4) Introduction of a thermoplastic elastomer is carried out as a means of preventing fractures such as cracks in a container. Thus, crack prevention in a high-pressure container made of CFRP is not intended to suppress gas permeability by, for example, isolating a gas leaking from a liner.